The present section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Low-Dynamic-Range images (LDR images) are images whose luminance values are represented with a limited number of bits (most often 8 or 10). This limited representation does not allow correct rendering of small signal variations, in particular in dark and bright luminance ranges. In high-dynamic range images (HDR images), the signal representation is extended in order to maintain a high accuracy of the signal over its entire range. In HDR images, pixel values are usually represented in floating-point format (either 32-bit or 16-bit for each component, namely float or half-float), the most popular format being openEXR half-float format (16-bit per RGB component, i.e. 48 bits per pixel) or in integers with a long representation, typically at least 16 bits.
A typical approach for encoding an HDR image is to reduce the dynamic range of the image in order to encode the image by means of a traditional encoding scheme (initially configured to encode LDR images).
According to a first approach, a tone-mapping operator is applied to the input HDR image and the tone-mapped image is then encoded by means of a traditional 8-10 bit depth encoding scheme such as JPEG/JPEG200 or MPEG-2, H.264/AVC for video (“Advanced video coding for generic audiovisual Services”, SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS, Recommendation ITU-T H.264, Telecommunication Standardization Sector of ITU, January 2012). Then, an inverse tone-mapping operator is applied to the decoded image and a residual image is calculated between the input image and the decoded and inverse-tone-mapped image. Finally, the residual image is encoded by means of a second traditional 8-10 bit-depth encoder scheme.
This first approach is backward compatible in the sense that a low dynamic range image may be decoded and displayed by means of a traditional apparatus.
However, this first approach uses two encoding schemes and limits the dynamic range of the input image to be twice the dynamic range of a traditional encoding scheme (16-20 bits). Moreover, such approach leads sometimes to a low dynamic range image with a weaker correlation with the input HDR image. This leads to low coding performance of the image.
According to a second approach, a backlight image is determined from the luminance component of the input HDR image. A residual image is then obtained by dividing the input HDR image by the backlight image and both the backlight image and the residual image are directly encoded.
This specific approach for encoding an input HDR image is not backward compatibility with a traditional apparatus which is not able to decode and/or display a high dynamic range.